A secondary battery has been recently used as a power source for driving mobile electronic equipment such as a notebook computer and a tablet terminal. A battery pack using such a secondary battery includes one that has a power saving state (off state) suppressing power consumption when working current is low to extend a battery life of the secondary battery besides a non-power saving state (on state) outputting normal discharging current. The timing of transition from such the power saving state to the non-power saving state is performed by, for example, detecting that a current amount required from electronic equipment on a load side becomes not less than a predetermined value. For example, in the notebook computer, when an interval of time during no usage has passed not less than a predetermined period, the notebook computer automatically transitions to a sleep mode (power saving state), and when a certain input such as a new keyboard operation is detected, the note book computer automatically returns from the sleep mode to normal operation (non-power saving state).
Appropriate returning from the power saving state to the non-power saving state in this manner needs to precisely detect a current (wake-up current) of not less than a predetermined value that becomes a threshold to return from the power saving state to the non-power saving state. The detection of the wake-up current is performed in a wake-up current detection circuit. To precisely detect the wake-up current by the wake-up current detection circuit, a zero point correction to correct a zero point of the wake-up current detection circuit (calibration operation) is needed. Such a calibration operation is typically performed during driving electronically driven equipment.
However, the wake-up current detection circuit has temperature characteristics, so that although current can be correctly detected with respect to a set value when the ambient temperature under which calibration operation has performed is maintained, a detection voltage disadvantageously fluctuates when the ambient temperature is changed, which can cause failure of detection at an intended current value. In this case, the wake-up current fails to be precisely detected, failing to detect a current value that should detect wake-up or causing, in an opposite manner, a detection of wake-up at a current value that should not be detected, which can result in failure of correct return from the power saving state to the non-power saving state.